1. Field of the Invention
The present invention relates to the treatment of silicon steel sheets, and more particularly to the treatment of silicate coatings on such sheets, in order to provide silicon steel sheets having electrically insulating protective coatings.
2. Description of the Prior Art
In the manufacture of silicon steel sheet material for use as so-called electrical sheets which have a grain orientation, after rolling and decarburization, the sheet material is conventionally heat treated at about 850.degree. to 1350.degree. C in order to achieve the necessary grain growth of the crystals, i.e., so that the sheet material will acquire the required magnetic properties. Before the heat treatment, however, the sheet material is usually coated with chemicals in order to produce, during the subsequent heat treatment, an electrically insulating protective coating.
One conventionally utilized coating has been a silicate or so-called "glass film" coating which consists of the reaction product of silicon dioxide and an alkaline earth metal oxide or hydroxide, the most common alkaline earth metal oxide or hydroxide being magnesium oxide or magnesium hydroxide. The coating is conventionally formed by applying to the surface of the sheet material a uniform layer of an aqueous suspension of the alkaline earth metal oxide or hydroxide and thereafter subjecting the sheet material to the noted heat treatment step, i.e., by subjecting the sheet material to a temperature of about 850.degree. to 1350.degree. C for several hours in a hydrogen atmosphere (the most optimum temperature being about 1000.degree. to 1350.degree. C in order to ensure that a well-developed glass film is formed). By these steps, the hydroxide, which is either in the suspension initially or which is formed from the oxide by reaction with water, liberates water as the temperature increases, and this water will, at temperatures below those mentioned, oxidize the silicon in the silicon steel sheet material to form silicon dioxide at the surface thereof. On the other hand, the iron itself will not be oxidized. The alkaline earth metal oxide, which is either still available from that in the original suspension or which is formed from the hydroxide after the liberation of water, then reacts with the silicon dioxide on the surface of the sheet material during the heat treatment to form the glass film. In another known process, the alkaline earth metal oxide or hydroxide can be replaced with an alkaline earth metal carbonate. In this alternative process the carbonate decomposes during heating to liberate carbon dioxide which then oxidizes the silicon on the sheet material to form silicon dioxide (without the oxidation of any iron), which then reacts the available alkaline earth metal oxide to form the glass film during the heat treatment.
In either of the noted processes, any excess unreacted oxide ultimately acts as a spacing material between adjacent layers of the sheet which are formed by turning the sheets into rolls or by use of the sheets as laminae in a stack. The excess oxide also helps prevent the layers from sticking or sintering together.
However, the conventional silicate coating as described above often has been found to have an insufficient electrical insulating resistance for many purposes to which the silicon steel sheets may be applicable, and as a result the protective coating has been often reinforced, either by treatment with phosphoric acid and metal phosphates, especially alkaline earth metal phosphates and aluminum phosphate (the older method), or by treatment with such solutions which also contain colloidal silica and chromic acid (this being the newer method) in order to form a further protective layer.
With regard to the latter-mentioned method, the incorporation of colloidal silica into the further protective coating results in improved insulation resistance, reduced dusting when the sheet material is machined, and favorable magnetostriction. On the other hand, the chromic acid is used to neutralize any excess phosphoric acids which are present either in the form of phosphoric acid in its original form from the original solution or else acids in a transformed state as a result of the heat treatment of the sheet material after the phosphate has been applied. The chromic acid functions due to the fact that at somewhat above 200.degree. C it thermally decomposes and forms chronium (III) ions which then react with the phosphoric acids to form chromium (III) phosphate. By neutralizing the phosphoric acids in this way, flaking off of the silicate layer, which is normally caused by the phosphoric acids during heat treatment, is prevented. In addition, the retention of phosphoric acids on the sheet material, which would normally occur to a significant extent due to the hindered evaporation thereof (as a result of the presence of the colloid silica) is prevented. In this regard, such undesirably retained phosphoric acids will normally diffuse out to the surface of the phosphate layer after the finished sheet has been stored for some time where it will take up water and become quite active in destroying the insulation coating.
However, it has now been discovered that even when the silicate layer is treated as noted above, the chromium (III) phosphate in fact can dissolve out from the phosphate layer if the sheet comes into contact with water (this occurs, for example, when the sheet material is handled or when it is used in a transformer where the oil is seldom entirely free of water), and this loss of chromium (III) phosphate can lead to a break in the layer such that its insulating ability is completely destroyed.
It is thus an object of the present invention to provide a method of neutralizing the excess phosphoric acids which are present on the protective layers overlying the silicate coatings on silicon steel sheets or objects in such a way that the formed phosphate compounds are either completely insensitive or else almost completely insensitive to water leaching.